Old-style JPEG compression in TIFFs are able to be read using Strip/Tile APIs. Although, it should be possible to read them using Scanline API, it does not work for some reason. Anyway, reading subsampled YCbCr formats through Strip/Tile/Scanline libtiff API does not de-subsample the data, so caller should unpack data to whatever format is appropriate. New-style JPEG compressed images were already read through libtiff as RGB images (https://github.com/python-pillow/Pillow/pull/3227). Unfortunately, there is no flag to ask libtiff to de-subsample old jpeg, but it provides a way to read any image as 32bit RGBA. This commit adds ability to read old-style JPEG TIFFs through reading *all* YCbCr images as RGBX using Tile and Strip reading API. This supersedes previous work (PR #3227) to read new-style JPEG-TIFFs.
JPEG-related tiff pseudo-tags are registered only when tiff image has jpeg compression. Trying to set TIFFTAG_JPEGCOLORMODE on non jpeg-compressed file prints error "Unknown pseudo-tag 65538".
